Cyber thieves have stolen millions of dollars from banks, cybersecurity and anti-virus provider Kaspersky Lab revealed Wednesday. At least 29 Russian banks were affected in 2015, and the hackers are still active and on the prowl for new victims, the Moscow-based company said.

Kaspersky Lab said it identified three different groups of hackers that "inflicted multiple millions in terms of financial damage to the banks." The names of the targeted banks weren't revealed.

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In 2015, cyber thieves infected a number of bank computers with phishing emails sent to employees that contained "malicious executable files" or by "targeting browser vulnerabilities." Once they had access to a bank's network, they used software to infiltrate its computers until they found one with access to financial transactions.

Any time one of the cyber thieves used a card from a hacked bank in another bank's ATM, the infected system "rolled back" the transaction, meaning the withdrawn cash was never deducted from the overall balance.

"As far as we know, the gang is relatively small and consists of up to ten people," Kaspersky Lab said Wednesday. "The hackers are still active and looking for new victims."